1. Field of the Invention
The present invention generally relates to supervisory control systems, and more particularly, to a supervisory control system which uses a cache function in a server system on the World Wide Web (WWW), or the like, to cache response information.
2. Description of the Related Art
FIG. 1 shows a conventional supervisory control system. In this supervisory control system, a supervisory control apparatus 12 supervises and controls a plurality of supervised apparatuses 13 via a supervisory control network. This supervisory control apparatus 12 is connected to supervisory control terminals 11 each having a browser function by a LAN (the supervisory control terminals 11 will be hereinafter referred to as xe2x80x9cbrowsers 11xe2x80x9d). The supervisory control apparatus 12 performs HTTP (Hyper Text Transfer Protocol) communication with the browsers 11 using a Web server function. The supervisory control apparatus 12 analyzes a control request (a supervisory control request) sent from the browsers 11, and then issues a command to the supervised apparatuses 13, thereby supervisory-controlling the supervised apparatuses 13. Response information sent from the supervised apparatuses 13 in reply is supplied to the browsers 11, which display the contents of the received response information.
The conventional supervisory control apparatus 12 using the Web server issues a command to the supervised apparatuses 13 to inquire for the supervisory control conditions every time a control request is sent by a browser 11 even for response information which does not change with time. Accordingly, every time a supervisory control request is made from a browser, the supervisory control apparatus 12 receives the corresponding response information from the supervised apparatuses 13 and transfers it to the browser. However, a larger amount of response information received from the supervised apparatuses 13 takes the supervisory control apparatus 12 longer to transfer it to the browser 11. This causes the browser 11 a delay in displaying the page.
Meanwhile, the Web server of the supervisory control apparatus 12 has a cache function. Upon receipt of supervisory information from the supervised apparatuses 13, the Web server with the cache function transfers the supervisory information to the browsers 11 and caches the supervisory information in a cache memory. When one of the browsers 11 inquires again for the same supervisory information, supervisory control apparatus 12 can read out the supervisory information from the cache memory and send it, instead of inquiring directly to the supervised apparatuses 13.
The response information from the supervised apparatuses 13 includes not only the information that does not change with time but also information which changes every time a page is displayed on a browser 11. In the prior art, each browser 11 has no means to judge whether response information sent from the supervisory control apparatus 12 is information obtained by inquiring directly from the supervised apparatuses 13 or cache information read out from the cache memory. As a result, response information not reflecting the current conditions of the supervised apparatuses 13 is sent to the browser 11. Also, the operator of the browser 11 may wrongly use information because he/she cannot judge whether the contents of a page displayed on the browser 11 are cache information or information obtained directly from the supervised apparatus 13.
A general object of the present invention is to provide supervisory control systems in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a supervisory control system which can quickly and accurately obtain supervisory control information from supervised apparatuses.
The above objects of the present invention are achieved by a supervisory control system which comprises a supervisory control apparatus, one or more supervised apparatuses connected to the supervisory control apparatus via a network, and a plurality of supervisory control terminals connected to the supervisory control apparatus via a network. The supervisory control apparatus issues a supervisory control instruction to the one or more supervised apparatuses upon receipt of a control request from a supervisory control terminal querying one or more supervised apparatuses, and supplies the supervisory control terminal with response information received as a response from the one or more supervised apparatuses.
In the above supervisory control apparatus, there are provided a cache storage unit which caches response information as cache information to be sent to one or more supervisory control terminals, and a cache management unit which manages cache information stored in the cache storage unit.
The supervisory control apparatus caches cache information in the cache storage unit. This cache information is the response information from a supervised apparatus in response to a supervisory control request from the supervisory control terminal. When the response information corresponding to the supervisory control request from the supervisory control terminal exists as cache information in the cache storage unit, the supervisory control apparatus then transmits the cache information to the supervisory control terminal. When the response information corresponding to the supervisory control request from the supervisory control terminal does not exist in the cache storage unit, the supervisory control apparatus issues a supervisory control instruction to the designated supervised apparatus to obtain response information, and transfers the thus obtained response information to the supervisory control terminal.
By determining whether response information from a supervised apparatus should be cached in the cache storage unit in accordance with a supervisory control request from the supervisory control terminal, only designated response information will be cached in the cache storage unit. Accordingly, in reply to a supervisory control request for response information which does not change with time, the supervisory control apparatus can promptly send the corresponding cache information to the supervisory control terminal. Meanwhile, response information which changes with the operating conditions of the supervised apparatuses is not cached in the cache storage unit. Thus, only highly reliable response information is sent to the supervisory control terminal.